Today I learned that bash has hashmaps (2024)
136 comments
·January 8, 2025teddyh
latexr
While I agree with the larger sentiment I think you’re making (I also make it a habit to peruse the manual just to see what is available), how often do you reread the manual, especially for your shell? I knew about associative arrays in bash, but not by reading the manual as those were introduced in v4 and I’ve been using bash for longer than that.
teddyh
You should probably read the release notes for new major versions. Also, anytime you think “I wish there was <feature> available in this program”, you should probably double-check if such a feature has appeared since you last learned the program.
dredmorbius
That's how I discovered the '&' regex feature in less some years back.
sgarland
Agreed. This is also how I found out that features in Python I took for granted, like breakpoint(), were much more recent than I thought (3.7). Nothing like having to use ancient versions of a language to discover limitations.
akvadrako
I do a fair bit of bash scripting and there is no way I can remember all the features and syntax I use, so of course I read parts of the manual regularly and often learn new things. You don't need to ever read the whole thing.
sgarland
I once worked with a dev (frontend, of all things) who reread the bash manual annually.
I work in ops, and I don’t even have that dedication. My hat is off to them.
dredmorbius
TBF, the Bash manpage now runs > 80 pages:
man -Tps bash | ps2pdf - bash.pdf
null
johannes1234321
Bash is low on the list of things to learn, especially as many greybeards suggest keeping on POSIX compatibility and/or using a "proper" language (Python, older: Perl) for anything longer than a few lines.
jeltz
They suggest that based on their experience. Error handling in bash is awful so I advice against using it for anything complex.
superq
You can go a really really long way, with a script that will work everywhere, with just set -e and aborting at the first error.
Better yet, 'bash unofficial strict mode':
set -euo pipefail
IFS=$'\n\t'irunmyownemail
Anything complex should be written in a competent language like Java. Script languages (like Bash and Python) are for short (a few lines long) scripts. Using the tool outside the scope of what it was designed for is not a good idea.
superq
Give bash some credit - it's actually amazing for even very large, complex systems (of which there are many -- quietly doing their jobs for decades.)
With xargs or GNU parallel, you can have multi-processing, too. Combining with curl or ffmpeg, you can literally build a production-grade web scraper or video transcoding pipeline in a couple of minutes and a few dozen lines of code.
dolni
I write a lot of shell and my advice is don't use plain POSIX shell. Write bash.
It is 2025. bash is present almost everywhere. If, by chance, your script is run somewhere that doesn't have bash then guess what?
Your POSIX script probably won't work anyway. It will be a different system, with different utilities and conventions.
Line count is not a good reason to choose or avoid bash. Bash is quite reliably the lowest common denominator.
I dare say it is even the best choice when your goal is accomplished by just running a bunch of commands.
All that said, bash is gross. I wish we had something as pervasive that wasn't so yucky.
wruza
My one-shot memory worsened with years but grasping concepts from examples got much better due to experience. So I find walls of text much less useful than a continuous “synopsis” of snippets and best practices. One usage example is worth a hundred of words and few pages of it may be worth a whole manual, especially when “at hand” through editor means. For bash idk if that exists on the internet, so I maintain my own in my one big text file. I wish every manual had the synopsis-only section (maybe I should just LLM these).
farrelle25
> One usage example is worth a hundred words ... For bash idk if that exists on the internet, ...
Totally agree with that - I maintain a big txt file too.
Maybe this bash compendium is a bit similar:
teddyh
You might like these:
• <https://learnxinyminutes.com/>
And some code search engines:
• Debian: <https://codesearch.debian.net/>
• Python: <https://www.programcreek.com/python/>
• Linux: <https://livegrep.com/search/linux>
• GitHub: <https://grep.app/>
• HTML, JS and CSS: <https://publicwww.com/>
syntacticbs
Oh dear. I've been trying to get people to not use this feature for a while.
One thing that has bitten me in the past is that, if you declare your associative arrays within a function, that associative array is ALWAYS global. Even if you declare it with `local -A` it will still be global. Despite that, you cannot pass an associative array to a function by value. I say "by value" because while you can't call `foo ${associative_array}` and pick it up in foo with `local arg=$1, you can pass it "by reference" with `foo associative_array` and pick it up in foo with `local -n arg=$1`, but if you give the passed in dereferenced variable a name that is already used in the global scope, it will blow up, eg `local -n associative_array=$1`.
As a general rule for myself when writing bash, if I think one of my colleagues who has an passable knowledge of bash will need to get man pages out to figure out what my code is doing, the bash foo is too strong and it needs to be dumbed down or re-written in another language. Associative arrays almost always hit this bar.
jpavel2
I'm not seeing this local scope leak with bash 5.2.15. The below script works as I'd expect:
#!/bin/bash
declare -A map1=([x]=2)
echo "1. Global scope map1[x]: ${map1[x]}"
func1() {
echo " * Enter func1"
local -A map1
map1[x]=3
echo " Local scope map1[x]: ${map1[x]}"
}
func1
echo "2. Global scope map1[x]: ${map1[x]}"
1. Global scope map1[x]: 2
* Enter func1
Local scope map1[x]: 3
2. Global scope map1[x]: 2syntacticbs
The local scope leak seems to only happen when you drop down the call stack. See below how I can call func2 from the top level and it's fine, but if I call it from within func1, it will leak.
#!/bin/bash
declare -A map1=([x]=2)
echo "1. Global scope map1[x]: ${map1[x]}"
func1() {
echo " * Enter func1"
local -A map1
map1[x]=3
echo " Local scope map1[x]: ${map1[x]}"
func2
}
func2() {
echo " * Enter func2"
echo " Local scope map1[x]: ${map1[x]}"
}
func1
func2
echo "2. Global scope map1[x]: ${map1[x]}"
UPDATE: I did a bit of exploration and it turns out ANY variable declared `local` is in the scope of a function lower down in the call stack. But if you declare a variable as `local` in a called function that shadows the name of a variable in a callee function, it will shadow the callee's name and reset the variable back to the vallee's value when the function returns. I have been writing bash for years and did not realise this is the case. It is even described in the man page: When local is used within a function, it causes the variable name to have a visible scope restricted to that function and its children.
Thank you. You have taught me two things today. One is a bash feature I did not know existed. The second is a new reason to avoid writing complex bash.
layer8
This is know as dynamic scope, as opposed to lexical scope: https://en.wikipedia.org/wiki/Scope_(computer_science)#Lexic...
aunderscored
Not only do they exist, but they have some fantastic foot guns! https://mywiki.wooledge.org/BashPitfalls#A.5B.5B_-v_hash.5B....
forgotpwd16
>they have some fantastic foot guns!
Otherwise wouldn't be getting the full shell experience.
miohtama
It's not like we would have 50 years of good programming language design and then stick to things that were created 50 years ago.
echelon
Why hasn't shell scripting evolved? It's god-awful.
Analemma_
- bash is available pretty much everywhere, so if you learn it you can always use it, whereas if you learn a weird new shell you'll be occasionally forced to fall back on bash anyway, so people learn just bash for efficiency's sake (because learning one shell is painful enough as it is). And any proposed replacement will be non-portable.
- some of the things that make shell scripting terrible can't be fixed in the shell itself, and need changes to the entire ecosystem of console applications. e.g. it would be awesome if every Unix utility output structured data like JSON which could be parsed/filtered, instead of the soup of plaintext that has to be memorized and manipulated with awk, but that almost certainly won't happen. There's a bunch of backward-compatibility requirements like VT-100 and terminal escape sequences limiting the scope of potential improvements as well
- there's a great deal of overlap between "people who do a lot of shell scripting" and "people who are suspicious of New Stuff and reluctant to try it"
cvz
It's evolved quite a bit if you don't need a `sh`-compatible shell.
I'm a huge fan of `fish` for personal scripting and interactive use: https://fishshell.com/
Obviously you'll still have `bash` installed somewhere on your computer, but at least you don't have to look at it as much.
vessenes
What would you like to see?
I’d guess there’s a solution to almost any set of priorities you have for shell scripting.
The domain space is extremely challenging: by default, executing any program on behalf of the caller using arbitrary text inputs, and interpreted.
All modern shells allow calling of literally any binary using the #!/usr/bin/env randombinary incantation.
Upshot: bash has its place, and while some of that place is unearned inertia, much of it is earned and chosen often by experienced technologists.
In my case, if I’m doing a lot of gluing together of text outputs from binaries, bash is my go-to tool. It’s extremely fast and expressive, and roughly 1/4 the length of say python and 1/8 the length of say go.
If I’m doing a lot of logic on text: python. Deploying lots of places/different architectures: go
Too
Evolve into what? Something that works and doesn't have foot guns? Then you might as well just use any other programming languages that exist today. I will not suggest any alternative as that just attract flame wars.
Why hasn't using a rock as a tool evolved? Exactly, because it's god-awful and got superseded, use a real hammer or a cordless screwdriver instead.
Just like the post about improvements to C on the front page today, that list can be made infinite. Language designers have already learned from those mistakes and created zig, rust, go or even c++, that fixes all of them and more. Fixing all the flaws will turn it into a different language anyhow. There is only that much you can polish a turd.
azriel91
I use this as my main shell on Windows, and as a supplementary on Mac and Linux.
rqtwteye
"fantastic foot guns"
It wouldn't be a worthy bash feature if after learning about it you wouldn't spend a few days figuring out why the damn thing doesn't work the way it works in any other language.
aunderscored
Indeed! It's delightful like all of bash. I've used them in a few places but I try not to
chengiz
Too many. I still write /bin/sh syntax (I know it's a symlink to bash now but I mean the old school sh). Anything that requires bash-that-isnt-sh is usually better written in perl or something else.
TristanBall
Only on some distro's.
Debian variants tend to link dash, which self consciously limits itself to posix compatibility.
stefankuehnel
Interesting, definitely need to keep that in mind.
wruza
A comment worth more than a post, thanks!
somat
I love shell, I think it's killer feature are pipes and whoever figured out how to design so you can pipe in and out of control structures(Doug Mcilroy?) is a goddamn genius. however, after writing one to many overly clever shell scripts, I have a very clearly delineated point in which the script has become too complex and it is time to rewrite in a language better suited for the task. and that point is when I need associative arrays.
A lot of the sins of shell are due to it's primary focus as an interactive language. Many of those features that make it so nice interactively really hurt it as a scripting language.
DSpinellis
Pipe-related concepts in various restricted forms were floating around for years. Doug McIlroy indeed proposed them in 1964 and was heading the Bell Labs team when they were implemented in the Third Research Edition of Unix (1973).
emmelaich
Sure, but in and out of control structures? I think that's the major point your parent was making.
orbisvicis
Control structures? Do you mean something like '{ cmd1 || cmd2; } | cmd3', where the control structures are the braces?
layer8
“Control structure” normally refers to conditionals and loops. You can pipe in and out of a for loop in the shell, maybe that’s what they mean.
DSpinellis
I advocate the following rules for when to write and when not to write a shell script.
# Write a shell script:
* Heavy lifting done by powerful tool (sort, grep, curl, git, sed, find, …)
* Script will glue diverse tools
* Workflow resembles a pipeline
* Steps can be interactively developed as shell commands
* Portability
* Avoid dependency hell
* One-off job
# Avoid shell scripting:
* Difficult to see the preceding patterns
* Hot loops
* Complex arithmetic / data structures / parameters / error handling
* Mostly binary data
* Large code body (> 500 LoC)
* Need a user interface
A need for associative arrays (implemented in Bash as via hashmaps) moves the task to the second category (avoid shell scripting).
nativeit
I’ve been a sysadmin for nearly a decade, a frontend web designer for much longer (I still have a book on all the exciting changes in HTML4), and while I can very easily learn, compose, and use many markup and scripting languages, I have always struggled with full-on programming languages, and I’m not exactly sure why. Part of it, I think, is that most tutorials and online learning resources are focused on novices who don’t have any existing grasp on general programming concepts and syntax, but I’m already pretty deep into Bash. To the extent that I am sure I have crossed the thresholds you list, and used quite long and complex Bash scripts for tasks that would almost certainly be easier in Python. I’d love to find A Bash Scripter’s Guide To Python or something similar—an intermediate course that assumes that I already know about variables, operators, functions, Boolean expressions, et al. I have searched for this a few times, but it’s full of keywords that makes searching Google difficult.
So this has inspired me to Ask HN, I’m getting ready to post it with a reference to this discussion, but thought I’d start here: does anyone know of a good resource for learning Python (or Go, Perl…any good small project/scripting/solo hacking languages) that’s tailored for folks who already have a good foundation in shell scripting, markup, config/infra automation, build tools, etc.? I’m open to books, web-based tutorials, apps, or just good examples for studying.
I’m open to the notion that I simply haven’t put in the work, and powered through the doldrums of novice tutorials far enough to get to where the meaty stuff is. Worst case, I’m a big fan of taking courses at the local community college for my own edification, absent any specific degree program or professional certification. It would still necessitate a lot of remedial/redundant steps, but I can always help others through those chapters while filling in any of my own gaps. Day-to-day, I generally work alone, but I find such things are easier with others to help motivate and focus my efforts. Perhaps I have answered my own question, but even still, I appreciate any advice/recommendations folks might have.
brianpan
One of the differences between "general programming" and the kinds of coding that you have done is the /approach/. I'm not sure if that's even the right word, but there's a different set of concerns that are important.
My guess is that you can easily learn the syntax and that you have the logical and analytical skills, but the part you have to learn is how to THINK about programming and how to DESIGN a program. If you have or haven't taking coding classes, I think reviewing topics like data structures, algorithms, encapsulation/object oriented programming, functional programming, etc. is the way to learn how to think about general programming. I don't think the language matters, but there might be resources about these topics in the language you're interested in.
An example of what you DON'T want is something like Effective Go (not just because it's out-of-date now): https://go.dev/doc/effective_go. This page can give you a really good base of information about what Go is, but I think you'll get much more bang for your buck with a resource that is more about the WHYs of programming rather than the WHATs.
nativeit
This was honestly extremely helpful. I very much appreciate the advice. I think you’re probably correct in that my intuition and prior knowledge is not being particularly helpful with shifting my paradigmatic perspectives, and while I may not need the novice lessons to learn the vocabulary or practical concepts, I probably do need a better formal introduction to theory. I am largely self-taught, aside from 2-years in an electronics engineering program that included embedded programming, and lessons in logic and Boolean algebra, My bachelor’s was in media studies and film production. In my experience, practical knowledge can be obtained through hands-on exercise and observing others—but theory and philosophy usually requires a teacher (or maybe months of meditation in a cabin in the woods, but as much as I admire Henry David Thoreau as an author, he wasn’t a great software engineer), and it sounds like that’s what I am missing here. Community college classes it is.
Thanks again!
nativeit
My search through existing Ask HN has shown that this, unsurprisingly, has been asked before by several people in other specific contexts (side note, Ask HN is such an incredibly useful resource, thanks to everyone who engages with these questions). I don’t want to add more noise, so I’m going to work through the existing answers before posting.
If anyone else is interested, this thread from 2020 is where I am starting, it seems to align with my own quest pretty well: https://news.ycombinator.com/item?id=22932794
wellshapedwords
> assumes that I already know about variables, operators, functions, Boolean expressions, et al.
Learning Go by Jon Bodner is a good choice. It seems to assume that Go is the reader's second (or tenth) language.
kmstout
A couple things:
- Between using Bash's functions and breaking large scripts into multiple, separate scripts, one can keep things reasonably tidy. Also, functions and scripts can be combined in all the ways (e.g., piping and process substition) that other programs can.
- If I run into a case where Bash is a poor fit for a job, I ask myself, "Self, what program would make this easy to do in a shell script?" If I can respond with a good answer, I write that, then continue with the shell scripting. If not, I write in something else (what the kids would call a "proper" language).
DiabloD3
No one else seems to have mentioned this, but POSIX sh does not include this feature.
Until it does, and major POSIX shs have shipped with it for a decade, then the feature will actually exist in a way that the average shell coder cares about. You're better off just shipping Rust/Go/etc binaries and leave sh for your simple glue tasks.
Even I've eventually switched to this, and I've written 10k+ long Bash scripts that followed best practices and passed shellcheck's more paranoid optional tests.
Use the right language for the right job.
abathur
> then the feature will actually exist in a way that the average shell coder cares about
I think it's worth picking at this a bit. At least IME, a fairly small fraction of the Shell I write needs to run literally (any|every)where.
I don't mean to suggest there aren't people/projects for whom the opposite is true--just that it's worth chunking those cases and thinking about them differently.
It obviously isn't a global solution, but in the Nix ecosystem we have decent idioms for asserting control over the shell and tools any given script runs with.
Reliable environment provisioning can spare hundreds of lines of code real-world Shell tends to accumulate to sanity-check the runtime environment and adapt to the presence/absence of various utilities. It also enables us to use newer shells/features (and shell libraries) with confidence.
It's enough leverage that I only give it up when I must.
nmz
POSIX sh doesn't even have arrays. I remember having to use awk for that functionality.
nielsbot
Nice that these exist—but does anyone else absolutely abhor shell programming? The syntax is impossible to memorize, it’s incredibly easy to make mistakes, and debugging is a pain. I hate it more than C++ and AppleScript.
ryapric
I enjoy it quite a lot :shrug:
Bash itself isn't a very big language, so I wouldn't call it "impossible to memorize".
irunmyownemail
I have around a billion short bash scripts, it's rare I've used an array but it's cool it has it as long as the script doesn't go beyond a few lines.
layer8
I find the semantics (e.g. the order in which elements are replaced/parsed/processed) more of a problem than the syntax.
nmz
Have you tried plan9's rc? easy to memorize syntax thing. It's still easy to make mistakes though and nobody uses it.
akvadrako
It has some rough edges but bash has the world's best REPL and along with -x that makes debugging quite easy.
oniony
A couple of weeks ago I learnt that Bash on Mac does not have associative arrays. We worked around the issue by changing the script to run under Zsh, but beware.
teddyh
Many programs on macOS are stuck in the ancient past, due to Apple:
<https://web.archive.org/web/20240810094701/https://meta.ath0...>
ptdorf
Sounds like a bash 3 issue.
$ bash --version
GNU bash, version 5.2.32(1)-release (aarch64-apple-darwin23.4.0)
$ declare -A aaa; aaa[a]=a; aaa[b]=bb; for i in ${!aaa[@]}; do echo "$i --> ${aaa[$i]}"; done
b --> bb
a --> a
forgotpwd16
To elaborate on this, macOS default bash is still stuck (assuming due to licensing) in v3.2.x (released in 2007). Bash got associative arrays in v4 (released in 2009).
x3n0ph3n3
You should be getting bash from homebrew anyways.
eadmund
Or using an OS which doesn’t ship ancient software. Linux exists. It’s pretty awesome.
wruza
Only until you find that your requirement or use case doesn’t work in it.
tgv
There's a time and place for evangelism. This isn't it.
x3n0ph3n3
When they make a decent laptop for linux, maybe I'll switch. I wanted so badly to move to System76, but their screens are awful.
oniony
My employer does not let me install Linux on my Mac.
dghf
Picky and probably pointless question: are they actually hashmaps? If I understand correctly, a hashmap isn’t the only way to implement an associative array.
meltyness
This seems pertinent, NEWS > bash-5.0 to bash-5.1 > mm
Seems to suggest it's indeed a hashmap in implementation, but I can't be bothered to look any closer.
zabzonk
That is certainly true - associative arrays in c++ (std::map) for example, are implemented as red-black trees.
tgv
I think many devs don't know the difference and simply call any dictionary/associative array a hash map. It might be one of the concepts that "the internet" promotes: it sounds fancy, more technical, makes you seem more knowledgeable, so it gets repeated. Then newcomers think this is the way it's always been called, and that gives it enough visibility to become the preferred name.
nmz
Quite frankly I'd love a programmers dictionary over terms, I recently called a file with a csv like format a db because, well, its not a csv and I don't know what else to call it.
layer8
…and if so, do they mitigate against hash collision attacks?
lars512
Unfortunately, MacOS ships an earlier version of bash that does not include associative arrays, so they’re not as portable as you might like.
fastily
You can use homebrew to install the latest version of bash and then chsh to set it as your default shell. That’s what I do anyways
PeterWhittaker
bash associative arrays are fantastic, I've made heavy use of them, but be warned that there have been several memory leaks in the implementation. (Sorry, no version notes, once we realized this, we rewrote a key component in C.)
IIRC, the latest bash addresses all of these, but that doesn't help too much if you are stuck with an older, stable OS, e.g., RHEL 7 or 8; even 9 likely has a few remaining.
These leaks become an issue if you have a long running bash script with frequent adds and updates. The update leak can be mitigated somewhat by calling unset, e.g., unset a[b], before updating, but only partially (again, apologies, no notes, just the memory of the discovery and the need to drop bash).
I'd agree with the idea that bash wasn't the best choice for this purpose in the first place, but there was history and other technical debt involved.
biorach
Every few years I rediscover this fact and every few years I do my best to forget it
It’s amazing how much of a superpower merely reading the manual is nowadays.
<https://www.gnu.org/software/bash/manual/bash.html#Arrays>